The reaction to convert carbon monoxide and hydrogen mixtures (defined herein as synthesis gas) to hydrocarbons over metallic catalysts has been known since the turn of the century. This reaction is commonly referred to as the Fischer-Tropsch or F-T synthesis. The synthesis gas used as feed to the process can be obtained from any source known to those skilled in the art, such as, for example, steam reforming of natural gas or partial oxidation of coal.
An important criterion for commercial F-T synthesis is having the ability to control the temperature of the reactants. The F-T reaction is highly exothermic. The efficient and rapid removal of heat is a major consideration in the generation of high molecular weight hydrocarbons. Unfortunately, high temperatures, i.e. above 325.degree. C., often lead to methane generation, carbon deposition on the catalyst, and catalyst particle fragmentation. Methane generation is usually not desired because the yield of higher hydrocarbons is reduced. Carbon deposition and catalyst particle fragmentation is undesirable because the catalyst life is shortened.
The prior art addressed the heat generation problem of the highly exothermic F-T reaction by using long tubular reactors that have a greater surface area to volume ratio than more conventional cylindrical reactors, thereby utilizing the additional surface area for cooling. Another method used by prior art is to run the reaction at low conversion rates per pass through the reactor, thereby using the unreacted gas to remove heat.
The temperature gain may also be controlled by utilizing a slurry bed reactor. The major drawbacks to the commercialization of the slurry bed reactor processes in the prior art are the separation of wax products and fine catalyst particles, and the mechanical failure due to high erosion of pump equipment used to re-circulate the slurry to the reactor zone.
Another problem with F-T commercialization is efficient conversion of reactants. The F-T synthesis generally utilizes hydrogen and carbon monoxide at a molar ratio of just over 2.0:1. Stoichiometrically, one hydrogen molecule combines with the carbon to form hydrocarbon and a second hydrogen molecule combines with the oxygen to form water vapor. The gas in the reactor can become depleted in one reactant, which will slow the reaction rate to levels below commercial viability. The reduction in the reaction rate is exacerbated when the reactant that is deficient in hydrogen. There are many different catalysts available for an F-T synthesis, and the effect of a deficiency in a reactant on the reaction rates varies among these catalysts. However, the effect of a deficiency in hydrogen gas is always more pronounced than is the effect of a deficiency in carbon monoxide. For instance, the F-T reaction rate with a cobalt based catalyst increases with both the partial pressure of hydrogen and of carbon monoxide; however, changes in the partial pressure of hydrogen have almost twice the effect as changes in the partial pressure of carbon monoxide. The reaction rate therefore drops off twice as fast when the synthesis gas is deficient in hydrogen as compared to the reaction rate decline caused by a deficiency in carbon monoxide.
This is usually not problematic if the ratio of hydrogen to carbon monoxide in the synthesis gas is about 2.1:1 and there is little methane synthesis. However, to obtain a greater fraction of waxy hydrocarbon product, a hydrogen to carbon monoxide ratio below a 2.1:1 ratio may be required. Therefore, as an F-T synthesis of waxy hydrocarbons reaction proceeds and the synthesis gas is converted to hydrocarbons, the synthesis gas can become progressively depleted in hydrogen. This results in a substantial portion of the hydrogen gas, carbon monoxide gas, or both, leaving the reactor without being converted.
Another related problem is that as the reactant gases become converted into hydrocarbons and water, diluent gases in the feed gas stream, e.g. water vapor, light hydrocarbons, and contaminants, may dilute the hydrogen gas and the carbon monoxide gas to the point that the reaction rate is significantly reduced. This further exacerbates the reduction in rate that is experienced when a reactant becomes deficient.
Finally, in typical reactors where a reactant is gas, the gas distribution in the reaction zone and back mixing are primary factors that determine reactor performance. Poor gas distribution will result in slug flow in slurry reactors or channeling in tubular reactors that results in reactant gases not uniformly exposed to catalyst. Gas maldistribution may result in a hot spot in the reactor which favors the undesired methanation reaction and may damage the catalyst. Back mixing will reduce the kinetic performance. Such maldistribution and back mixing often occur in conventional F-T processes.
It would be desirable if an F-T process could be developed which provides sufficient temperature control so as to avoid methanation, catalyst deactivation through carbon deposition, and catalyst fragmentation. It would further be desirable if such a process offered a manner of maintaining a hydrogen to carbon monoxide ratio other than the stoichiometric ratio of 2.1:1. It would still further be desirable if such a process offered a means of distributing the reactant gases in the catalyst bed in such a manner to reduce channeling or back-mixing.